docs: def apt_mark(packages, mark, fatal=False):
      """Flag one or more packages using apt-mark."""
      log("Marking {} as {}".format(packages, mark))
      cmd = ['apt-mark', mark]
      if isinstance(packages, six.string_types):
          cmd.append(packages)
      else:
          cmd.extend(packages)
  
      if fatal:
          subprocess.check_call(cmd, universal_newlines=True)
      else:
          subprocess.call(cmd, universal_newlines=True)